CN107110407B

CN107110407B - Threaded joint for steel pipe

Info

Publication number: CN107110407B
Application number: CN201580053616.8A
Authority: CN
Inventors: 杉野正明; 西川幸弘
Original assignee: Vallourec Oil and Gas France SAS; Nippon Steel and Sumitomo Metal Corp
Current assignee: Vallourec Oil and Gas France SAS; Nippon Steel Corp
Priority date: 2014-10-06
Filing date: 2015-10-05
Publication date: 2020-04-17
Anticipated expiration: 2035-10-05
Also published as: EP3205918B1; JP6588453B2; BR112017005668B1; CA2961189C; RU2659834C1; US20170292638A1; WO2016056222A1; EP3205918A1; EP3205918A4; CN107110407A; AR102507A1; US10400922B2; BR112017005668A2; JPWO2016056222A1; SA517381243B1; MX2017004325A; CA2961189A1

Abstract

The invention provides a threaded joint for a steel pipe. The threaded joint comprises a male buckle part (10) and a female buckle part (20), wherein the outer diameter of the female buckle part (20) is smaller than 104% of the outer diameter of the pipe main body of the male buckle part (10). The female snap part (20) has a lip part (24) and an internal thread part (21) in this order from the tip end toward the tube main body, and the lip part (24) includes a shoulder surface (22) and a sealing surface (23). The male snap portion (10) includes a shoulder surface (12), a sealing surface (13), and an external threaded portion (11). The shoulder surfaces (12, 22) are inclined from a plane perpendicular to the pipe axis (CL) toward the screwing-in traveling direction of the male buckle section (10). A lip portion (24) of the female buckle portion (20) has a protruding portion (25) disposed between the shoulder surface (22) and the sealing surface (23), and an annular portion (26) disposed between the sealing surface (23) and the female screw portion (21). The protruding portion (25) and the annular portion (26) do not contact the male buckle portion (10), and the length of the annular portion (26) is longer than the pitch of the female thread portion (21).

Description

Threaded joint for steel pipe

Technical Field

The present invention relates to a threaded joint for joining steel pipes.

Background

In Oil wells, gas wells, and the like (hereinafter, also referred to as "Oil wells"), steel pipes called Oil Country Tubular Goods (OCTG) are used to extract underground resources. The steel pipes are connected in sequence, and the connection uses a threaded joint.

The forms of threaded joints for steel pipes are roughly classified into a coupling (coupling) type and an integral (integral) type. In the case of the collar type, one of a pair of pipes to be connected is a steel pipe, and the other pipe is a collar. In this case, male screw portions are formed on the outer peripheries of both end portions of the steel pipe, and female screw portions are formed on the inner peripheries of both end portions of the coupling. Then, the steel pipe and the coupling are coupled. In the integral type, both of the pair of pipes to be connected are steel pipes, and no separate collar is used. In this case, an external thread portion is formed on the outer periphery of one end portion of the steel pipe, and an internal thread portion is formed on the inner periphery of the other end portion. Then, one steel pipe and the other steel pipe are joined.

Further, the form of the threaded joint for steel pipes is sometimes classified into a flat (flush) type, a semi-flat (semi flash) type, a slender (slim) type, and the like, depending on the outer diameter of the joint portion. The elongation is defined by using, as an index, a coupling (hereinafter, also referred to as a "standard coupling") having a cross-sectional area of a dangerous cross-section substantially equal to that of a pipe body. Specifically, the slim type refers to a threaded joint in which the maximum outer diameter of the joint part is smaller than the outer diameter of a standard collar. The flat type and the semi-flat type are defined by using the tube body as an index. Specifically, the flat type refers to a threaded joint in which the maximum outer diameter of the joint portion is substantially equal to the outer diameter of the pipe main body. A semi-planar type refers to a threaded joint where the maximum outer diameter of the joint part is between the maximum outer diameter of the planar joint part and the maximum outer diameter of the elongated joint part. In short, the joint portion is formed into a flat surface type, a semi-flat surface type, and a slender type in order from the point where the maximum outer diameter of the joint portion is small.

The dangerous cross section referred to herein is a cross section at a position where a cross sectional area subjected to a tensile load is smallest in an end portion of an engagement region of the threaded portion. The position of the dangerous cross section and the cross-sectional area thereof are factors for determining the tensile strength of the threaded joint. There is no definition that clearly distinguishes planar, semi-planar, and slim. However, among threaded joints for steel pipes currently in circulation, a threaded joint for steel pipes in which the maximum outer diameter of the joint portion is about 104% of the outer diameter of the pipe body is generally called a flat-type or semi-flat-type threaded joint, and a threaded joint for steel pipes in which the maximum outer diameter of the joint portion is about 108% of the outer diameter of the pipe body is generally called an elongated-type threaded joint.

In general, a joint portion of a pipe end portion formed with an external thread portion includes an element inserted into an internal thread portion, and is therefore called a male pin portion (pin). On the other hand, the joint portion of the pipe end portion formed with the female screw portion includes an element for receiving the male screw portion, and is therefore called a box (box). The male and female snap portions are ends of the tubing and are therefore both tubular.

In a use environment, a threaded joint for steel pipes receives high pressure from fluids (e.g., gas, liquid) present outside and inside. Hereinafter, the pressure from the outside is also referred to as external pressure, and the pressure from the inside is also referred to as internal pressure. Under such circumstances, a threaded joint for steel pipes is required to have high sealing performance against external pressure and internal pressure.

A threaded joint for steel pipes employs a tapered thread as a thread formed by an external thread portion of a pin portion and an internal thread portion of a box portion. In a threaded joint using a tapered thread, an external thread portion of a male thread portion and an internal thread portion of a female thread portion are fitted and sealed. Thereby forming a thread seal. Furthermore, in order to enhance the sealing performance of the thread seal, an outer seal or an inner seal is provided independently of the thread seal.

The outer seal includes a sealing surface formed on an inner periphery of a tip end portion of the box portion and a sealing surface formed on an outer periphery of the pin portion in correspondence therewith. The outer seal is formed by the sealing surfaces interfering with each other and coming into contact with a high surface pressure. The outer seal mainly functions to prevent external fluid from accidentally entering the region of the thread seal, and helps ensure sealing performance against external pressure.

The inner seal includes a seal surface formed on the outer periphery of the tip end portion of the pin portion and a seal surface formed on the inner periphery of the box portion in correspondence therewith. The inner seal is formed by the sealing surfaces interfering with each other and coming into contact with a high surface pressure. The inner seal mainly functions to prevent the fluid inside from accidentally entering the region of the thread seal, and helps ensure sealing performance against the internal pressure.

Typically, in the region of the outer seal, the wall thickness of the female portion is thinner than the wall thickness of the male portion. Therefore, when the threaded joint is subjected to a high internal pressure and the entire threaded joint is expanded in the radial direction, the outer seal region of the box portion is easily expanded in diameter and plastically deformed. In this case, even if the internal pressure is removed, the outer seal region of the box portion is kept in a state of being expanded in diameter, and therefore, the contact surface pressure between the seal surfaces of the outer seal is significantly reduced. In this state, when the threaded joint is loaded with a high external pressure, the sealing surfaces of the external seals are easily separated from each other. At this point, the fluid from the outside may dip beyond the outer seal into the area of the thread seal and eventually into the interior of the threaded joint. This undesirable condition is known as external pressure leakage.

Various techniques for ensuring sealing performance of the outer seal have been proposed. For example, in U.S. patent No. 7506900 (patent document 1), european patent application publication No. 2325435 (patent document 2), international publication No. 2009/083523 (patent document 3), international publication No. 2011/044690 (patent document 4), U.S. patent application publication No. 2010/181763 (patent document 5), and U.S. patent application publication No. 2008/265575 (patent document 6), techniques for increasing the contact surface pressure at the outer seal are disclosed.

In the techniques of patent documents 1 and 2, a protruding portion is provided at the distal end portion of the female snap portion. The protruding portion extends from the top end side of the outer seal sealing surface in the tube axis direction and does not contact the male snap portion. The deformation resistance of the outer sealing region is increased by the stiffness of such a projection itself. As a result, in the outer seal region of the box portion, plastic deformation of the expanded diameter is suppressed, and the contact surface pressure between the seal surfaces is ensured.

In the techniques of patent documents 3 to 5, a shoulder surface is provided at the tip of the box portion. The male buckle part is provided with a shoulder surface corresponding to the shoulder surface of the female buckle part. The shoulder surfaces of the female snap portion and the male snap portion are pressed against each other by the screwing of the male snap portion, and function as a stopper for restricting the screwing of the male snap portion. In addition, the shoulder surface of the pin portion and the shoulder surface of the box portion exert a so-called tightening axial force acting on the load flanks of the male thread portion of the pin portion and the female thread portion of the box portion, respectively, in a state in which the tightening is completed. The shoulder surfaces of the female and male snap portions are inclined in the screwing-in direction of the male snap portion from the plane perpendicular to the pipe axis, and press-contact is performed in a hook-like manner. By thus bringing the shoulder surfaces into press contact with each other in the hook-like form, the outer seal region of the female snap portion receives a reaction force in the direction of reducing the diameter. As a result, in the outer seal region of the box portion, plastic deformation of the expanded diameter is suppressed, and the contact surface pressure between the seal surfaces is ensured.

The technique of patent document 6 employs two techniques, namely, the technique of patent document 1 and patent document 2 (i.e., the technique of providing a protruding portion at the distal end of a box portion) and the techniques of patent documents 3 to 5 (i.e., the technique of providing a shoulder surface at the distal end of a box portion, the shoulder surface being in press contact with a pin portion in a hook-like shape).

Documents of the prior art

Patent document

Patent document 1: specification of U.S. Pat. No. 7506900

Patent document 2: european patent application publication No. 2325435

Patent document 3: international publication No. 2009/083523

Patent document 4: international publication No. 2011/044690

Patent document 5: U.S. patent application publication No. 2010/181763 specification

Patent document 6: U.S. patent application publication No. 2008/265575 specification

Disclosure of Invention

Problems to be solved by the invention

The techniques of patent documents 1 to 6 are effective in the case of a threaded joint in which the thickness of the box portion is thicker than the thickness of the pin portion, such as a collar-type threaded joint or a threaded joint used for a small-diameter steel pipe although it is slender. However, in the case of a threaded joint in which the expansion of the outer diameter of the box portion is restricted and the outer diameter of the box portion is approximately the same as the outer diameter of the pin portion, such as a flat-type threaded joint, a semi-flat-type threaded joint, or a slender threaded joint which is often used for large-diameter steel pipes, the effects of the techniques of patent documents 1 to 6 cannot be sufficiently exhibited, and external pressure leakage cannot be prevented. This is because the thickness of the female portion is significantly thinner than that of the male portion, and accordingly, the thickness of the protruding portion at the distal end portion of the female portion becomes thinner, and the width of the shoulder surface at the distal end of the female portion becomes narrower.

The present invention aims to provide a threaded joint for a steel pipe having a characteristic of ensuring sealing performance of outer seal reliably.

Means for solving the problems

An embodiment of the present invention provides a threaded joint for steel pipes including a tubular pin portion and a tubular box portion, the pin portion being screwed into the box portion and the pin portion and the box portion being fastened together, wherein,

the outer diameter of the female snap portion is less than 104% of the outer diameter of the tube body of the male snap portion.

The female snap portion has a lip portion and a tapered threaded internal threaded portion in order from the top end toward the pipe main body, the lip portion including a shoulder surface and a sealing surface.

The pin part includes: a shoulder surface that comes into contact with the shoulder surface of the female snap portion in a state in which fastening is completed; a sealing surface that comes into contact with the sealing surface of the female snap portion in a fastened state; and an external thread part which is a tapered thread and is fitted into the internal thread part in a fastened state.

The shoulder surfaces of the female snap portion and the male snap portion are inclined from a plane perpendicular to the pipe axis toward the screwing-in traveling direction of the male snap portion.

The lip portion of the female snap portion has a protruding portion disposed between the shoulder surface and the sealing surface and an annular portion disposed between the sealing surface and the female screw portion.

The protruding portion and the annular portion are not in contact with the male snap portion in a state where the fastening is completed,

the length of the annular portion in the pipe axis direction is longer than the pitch of the internal thread portion.

In the threaded joint, the pin portion may have an auxiliary seal surface at a distal end portion thereof, and the box portion may include an auxiliary seal surface that comes into contact with the auxiliary seal surface of the pin portion in a fastened state.

In the threaded joint, the pin portion may have an auxiliary shoulder surface at a distal end thereof, and the box portion may include an auxiliary shoulder surface that comes into contact with the auxiliary shoulder surface of the pin portion in a fastened state.

In the threaded joint, the male thread portion of the male buckle portion and the female thread portion of the female buckle portion may be two-step threads divided into two steps along a pipe axis. In the threaded joint, the male portion may have an intermediate seal surface between the male thread portion of the first step and the male thread portion of the second step, and the female portion may include an intermediate seal surface that comes into contact with the intermediate seal surface of the male portion in a state in which the fastening is completed. In addition, the male snap portion may have an intermediate shoulder surface between the male screw portion of the first step and the male screw portion of the second step, and the female snap portion may include an intermediate shoulder surface that comes into contact with the intermediate shoulder surface of the male snap portion in a state in which the fastening is completed.

In the threaded joint, it is preferable that the tensile yield strength in the circumferential direction in the region of both the protruding portion and the sealing surface of the box portion is 105% or more of the tensile yield strength in the region of the tube main body of the box portion.

In the threaded joint, the shape of the sealing surface of each of the box portion and the pin portion may be one of a straight line, an arc, an elliptic arc, and a quadratic curve, or a combination of two or more of these in a longitudinal section taken along a pipe axis.

The threaded joint may be configured such that the outer diameter of the pipe body of the male snap portion is 170mm or more.

ADVANTAGEOUS EFFECTS OF INVENTION

The threaded joint for a steel pipe according to the present invention has a significant effect of being able to reliably ensure the sealing performance of the outer seal.

Drawings

Fig. 1 is a longitudinal sectional view showing a threaded joint for steel pipes according to an embodiment of the present invention.

Fig. 2 is an enlarged longitudinal sectional view of a distal end portion of a box portion of the threaded joint for steel pipes shown in fig. 1.

Fig. 3 is an enlarged vertical cross-sectional view of a region of a threaded portion of the threaded joint for steel pipes shown in fig. 1.

Fig. 4 is a longitudinal sectional view showing another example of a taper thread applicable to a threaded joint for steel pipes according to an embodiment of the present invention.

Fig. 5 is a longitudinal sectional view showing still another example of a taper thread applicable to a threaded joint for steel pipes according to an embodiment of the present invention.

Fig. 6 is a longitudinal sectional view schematically showing a region of the seal based on the surface contact.

Fig. 7 is a longitudinal sectional view schematically showing a region of the threaded joint of a shape different from that of fig. 6, which is sealed by surface contact.

Detailed Description

As described above, in the threaded joint for steel pipes, the expansion of the outer diameter of the box portion is restricted. Therefore, the thickness of the box portion, particularly the thickness of the tip portion of the box portion, is naturally reduced. In the case of a threaded joint in which a protruding portion is provided at the distal end portion of the box portion, the thickness of the protruding portion becomes thinner as the distal end portion of the box portion becomes thinner. This makes it impossible to sufficiently secure the rigidity of the protruding portion itself. In the case of a threaded joint in which a shoulder surface that is in press contact with the pin portion in a hook-like manner is provided at the tip end of the box portion, the width of the shoulder surface is narrowed as the tip end portion of the box portion is made thinner. Thereby, a radial component of the reaction force of the shoulder surface of the box tip for increasing the contact surface pressure of the outer seal is restricted. Thus, in either case, the effect of increasing the contact surface pressure at the outer seal is limited.

Therefore, the present inventors have studied to make full use of the thinning of the distal end portion of the female snap portion. Specifically, the present inventors have conceived that, when a high external pressure is applied to a threaded joint, if the external seal region of the box portion can be pressed against the external seal region of the pin portion, the contact surface pressure at the external seal portion may be increased. As a result of intensive studies, the following structure was found to be effective.

In the form of the distal end portion of the female snap portion, an annular groove is formed adjacent to the female screw portion side of the sealing surface of the outer seal. By forming the annular groove, an annular portion having a small wall thickness is formed between the seal surface and the female screw portion. The length of the annular portion in the pipe axial direction is made longer to some extent. By providing the annular portion, the distance from the female screw portion to the sealing surface of the outer seal is extended.

With this configuration, when a high external pressure is applied to the threaded joint, the annular portion and the outer seal region continuous with the annular portion are reduced in diameter. Thus, the outer seal region of the female snap portion is pressed against the outer seal region of the male snap portion, and the contact surface pressure at the outer seal portion can be increased.

In short, a protrusion is provided at the tip end of the female snap portion, and a shoulder surface that comes into press contact with the male snap portion in a hook-like shape is provided at the tip end of the female snap portion. An annular portion is provided between the external sealing region of the box portion and the internal thread portion. By these synergistic effects, the increase of the interface pressure at the outer seal is significantly reflected. As a result, the sealing performance of the outer seal can be reliably ensured.

The threaded joint for steel pipes of the present invention is completed based on the above findings. Hereinafter, embodiments of the threaded joint for steel pipes according to the present invention will be described.

Fig. 1 is a longitudinal sectional view showing a threaded joint for steel pipes according to an embodiment of the present invention. Fig. 2 is an enlarged longitudinal sectional view of a distal end portion of a box portion of the threaded joint for steel pipes shown in fig. 1. Fig. 3 is an enlarged vertical cross-sectional view of a region of a threaded portion of the threaded joint for steel pipes shown in fig. 1. As shown in fig. 1 to 3, the threaded joint of the present embodiment is a one-piece threaded joint, and includes a pin portion 10 and a box portion 20. As a matter of course, the threaded joint of the present embodiment can also be applied to a collar-type threaded joint.

The threaded joint of the present embodiment is directed to a threaded joint in which the wall thickness of the distal end portion of the box portion 20 is thin. Therefore, the outer diameter of the female snap portion 20 is greater than 100% and less than 104% of the outer diameter of the pipe body of the male snap portion 10. The threaded joint of the present embodiment is not particularly limited to the size of the steel pipe to be connected, but the threaded joint of the present embodiment is particularly suitable for connecting large-diameter steel pipes in which the thickness of the distal end portion of the box portion 20 is reduced. The large-diameter steel pipe is a steel pipe in which the outer diameter of the pipe body of the male buckle portion 10 is 170mm or more.

The female snap portion 20 has a lip portion 24 and an internally threaded portion 21 in this order from the tip end toward the tube main body, the lip portion 24 including a shoulder surface 22 and a sealing surface 23. The lip 24 extends from the distal end side of the female screw portion 21 in the pipe axis CL direction. The lip portion includes a protruding portion 25 disposed between the shoulder surface 22 and the sealing surface 23, and an annular portion 26 disposed between the sealing surface 23 and the female screw portion 21. The ring-shaped portion 26 is not formed with the female screw portion 21.

The annular portion 26 extends from the distal end side of the female screw portion 21 in the pipe axis CL direction and is continuous with the seal surface 23. The annular portion 26 is formed by, for example, forming an annular groove adjacent to one side of the seal surface 23 close to the female screw portion 21. That is, as shown in fig. 2, the inner diameter of the annular portion 26 of the box portion 20 is larger than the minimum diameter of the seal surface 23 and the maximum diameter of the thread groove bottom surface 21b of the female thread portion 21.

The projection 25 extends from the tip side of the seal surface 23 in the tube axis CL direction. A shoulder surface 22 is provided at the tip of the protrusion 25 (corresponding to the tip of the lip 24 and the tip of the female snap portion 20).

The shoulder surface 22 is an annular surface inclined from a surface perpendicular to the pipe axis CL toward the screwing travel direction of the male buckle portion 10 (a direction toward the tip end of the male buckle portion 10). In other words, the shoulder surface 22 is inclined toward the tip side of the male snap portion 10 toward the outer peripheral side thereof. In a longitudinal section taken along the pipe axis CL, the shape of the shoulder surface 22 is a straight line. In a longitudinal section taken along the pipe axis CL, the shape of the land 22 needs to match the shape of the land 12 of the male snap portion 10 described later. The shape of the shoulder surface 22 may be a curved line as long as this condition is satisfied.

The seal surface 23 is tapered and provided on the inner periphery of the lip 24. The sealing surface 23 has a shape in which a surface corresponding to the circumferential surface of a truncated cone having a diameter that increases toward the distal end side (the shoulder surface 22 side) is formed, or a shape in which the circumferential surface of such a truncated cone is combined with a surface corresponding to the circumferential surface of a rotating body obtained by rotating a curve such as a circular arc around the pipe axis CL. In other words, in a vertical cross section taken along the pipe axis CL, the shape of the seal surface 23 is any one of a straight line, an arc, an elliptic arc, and a quadratic curve, or a combination of any two or more thereof.

On the other hand, the male snap portion 10 has a shoulder surface 12, a seal surface 13, and a male screw portion 11 in this order from the tube body side toward the distal end side. The shoulder surface 12, the sealing surface 13, and the male screw portion 11 of the male screw portion 10 are provided corresponding to the shoulder surface 22, the sealing surface 23, and the female screw portion 21 of the female screw portion 20, respectively.

The male thread portion 11 of the male snap portion 10 and the female thread portion 21 of the female snap portion 20 are both tapered threads and constitute mutually engaging thread portions. As shown in fig. 3, the male screw portion 11 of the male snap portion 10 includes a thread crest surface 11a, a thread groove bottom surface 11b, a stabbing flank surface 11c preceding in the screwing process, and a load flank surface 11d located on the opposite side from the stabbing flank surface. On the other hand, the female thread portion 21 of the box portion 20 includes a crest surface 21a opposing the crest surface 11b of the male thread portion 11, a crest surface 21b opposing the crest surface 11a of the male thread portion 11, a stab flank surface 21c opposing the stab flank surface 11c of the male thread portion 11, and a load flank surface 21d opposing the load flank surface 11d of the male thread portion 11. The tapered thread in the present embodiment is a dovetail type tapered thread. Thus, the

load flanks

11d, 21d and the stab flanks 11c, 21c each have a negative profile half angle.

The male screw part 11 of the male snap part 10 and the female screw part 21 of the female snap part 20 can be screwed into each other. When the fastening is completed, the thread groove bottom surface 11b of the male thread portion 11 and the thread ridge top surface 21a of the female thread portion 21 are in close contact with each other, and the

load flanks

11d, 21d are also in close contact with each other. In the fastened state, gaps are formed between the crest surface 11a of the male screw portion 11 and the groove bottom surface 21b of the female screw portion 21, and between the stabbing flank surfaces 11c, 21c, and the gaps are filled with the lubricant. Thereby, a thread seal is formed. The sealing surfaces 13 and 23 come into contact with each other as the male buckle portion 10 is screwed in, and interfere with each other and come into close contact with each other in a state where the fastening is completed, thereby achieving a state of interference fit. Thereby, an outer seal by surface contact is formed. The shoulder surfaces 12 and 22 are pressed into contact with each other in a hook-like manner as the male snap portion 10 is screwed in, and in a state where the fastening is completed, a fastening axial force is applied to the load flank surface 11d of the male snap portion 11 of the male snap portion 10.

In the fastened state, a gap is formed between the protrusion 25 of the female snap portion 20 and the male snap portion 10, and the protrusion 25 and the male snap portion 10 do not contact each other. Further, a gap is formed between the annular portion 26 of the box portion 20 and the pin portion 10, and the annular portion 26 and the pin portion 10 do not contact each other.

In the threaded joint for steel pipes of the present embodiment configured as described above, the deformation resistance of the region (outer seal region) of the seal surface 23 continuous with the projection 25 is increased by the rigidity of the projection 25 itself of the box portion 20. Further, since the shoulder surfaces 12 and 22 are pressed into contact with each other in a hook-like manner, the outer seal region of the box portion 20 receives a reaction force in the direction of reducing the diameter. As a result, in the outer seal region of the box portion 20, plastic deformation with increased diameter is suppressed, and the contact surface pressure between the seal surfaces 13 and 23 is increased.

When a high external pressure is applied to the threaded joint, the annular portion 26 of the box portion 20 and the outer seal region continuous with the annular portion 26 are reduced in diameter. Thus, in the external seal region, the seal surface 23 of the box portion 20 is pressed against the seal surface 13 of the pin portion 10, and the contact surface pressure between the seal surfaces 13 and 23 can be increased.

In this way, the effect of increasing the contact surface pressure at the outer seal is mutually enhanced. As a result, the sealing performance of the outer seal can be reliably ensured, and external pressure leakage can be prevented.

Further, the threaded joint of the present embodiment supplementarily includes an inner seal. Specifically, as shown in fig. 1, the male snap portion 10 includes an auxiliary seal surface 17 at its tip end portion. The female snap portion 20 includes a secondary sealing surface 27 corresponding to the secondary sealing surface 17 of the male snap portion 10. The auxiliary seal surfaces 17 and 27 come into contact with each other as the male buckle portion 10 is screwed in, and interfere with each other and come into close contact with each other in a state where fastening is completed, thereby achieving an interference fit state. Thereby, an inner seal based on surface contact is formed.

Hereinafter, preferred embodiments of the main portions will be described.

Projection part

When the length of the projection of the female snap in the pipe axis direction is too short, the rigidity of the projection itself is insufficient, and therefore, the effect of increasing the contact surface pressure at the outer seal portion cannot be effectively exhibited. On the other hand, if the length of the protruding portion is too long, the raw material cost increases by the amount of the extension, and the manufacturing cost also increases. Further, since the distance from the land surface to the seal surface becomes long, the reaction force from the land surface which is brought into press contact in the hook-like form no longer effectively acts on the outer seal region. Therefore, the length of the protruding portion is preferably 0.5 to 3.5 times the thickness of the protruding portion. A more preferred lower limit of the length of the protrusion is 1.5 times the wall thickness of the protrusion. A more preferable upper limit of the length of the protruding portion is 3.0 times the wall thickness of the protruding portion.

Sealing surface

In a state where fastening is completed, when the contact length of the seal surfaces with each other in the pipe axis direction is excessively short, the sealing performance is insufficient. On the other hand, if the contact length between the seal surfaces is too long, the average contact surface pressure decreases, and the sealing performance is insufficient. Therefore, the contact length between the seal faces is preferably 0.5mm to 5 mm. A more preferred lower limit of the contact length is 1 mm. A more preferred upper limit of the contact length is 3.5 mm.

Shoulder surface

When the front angle (angle of フック in japanese) (the inclination angle from the plane perpendicular to the pipe axis) of the shoulder surface is too small, the reaction force from the shoulder surface which is brought into press contact in the hook-like form becomes small. Therefore, the effect of increasing the contact surface pressure at the outer seal portion cannot be effectively exhibited. On the other hand, when the front angle is too large, the rigidity of the region of the male snap portion having the shoulder surface is lowered, and the shoulder surface region is easily plastically deformed, thereby also adversely affecting the outer seal. Therefore, the land surface preferably has a rake angle of 5 deg. to 25 deg.. A more preferred lower limit of the rake angle is 9 deg. A more preferred upper limit for the hook angle is 20 deg.

Annular part

As described above, when external pressure is applied, the annular portion of the female snap portion reduces the diameter of the outer seal region of the female snap portion, thereby increasing the contact surface pressure at the outer seal portion. The annular portion of the female snap portion functions as a tool for retracting a cutting tool during the thread cutting of the female screw portion by utilizing the internal space thereof.

If the length of the annular portion in the pipe axis direction is too short, it is difficult to ensure the amount of tool retraction of the cutting tool in addition to the insufficient diameter reduction of the outer seal region. Therefore, the length of the annular portion is longer than the pitch of the internal thread portion. More preferably, the length of the annular portion is 1.2 times or more the pitch of the internal thread portion. On the other hand, if the length of the annular portion is too long, the raw material cost increases by the amount of the extension, and the manufacturing cost also increases. Therefore, a preferable upper limit of the length of the annular portion is 4 times the pitch of the internal thread portion, and a more preferable upper limit of the length of the annular portion is 2.5 times the pitch of the internal thread portion.

Here, the threaded joint for steel pipes of the present embodiment preferably has the following configuration. As shown in fig. 1 and 2, the tensile yield strength in the circumferential direction in the region of both the protrusion 25 and the seal surface 23 of the box portion 20 is 105% or more of the tensile yield strength in the region of the pipe body of the box portion 20. It is more preferable that the tensile yield strength in the circumferential direction in the region of both the protrusion 25 and the sealing surface 23 of the box portion 20 is 110% or more of the tensile yield strength in the region of the pipe main body of the box portion 20. By thus locally increasing the tensile yield strength in the region of both the protrusion 25 and the seal surface 23 of the box portion 20, the deformation of the outer seal region of the box portion 20 in the expanded diameter can be suppressed. As a result, the effect of increasing the contact surface pressure between the seal surfaces 13 and 23 is further exhibited.

As a method of locally increasing the tensile yield strength, there is a method of: before the box portion 20 is subjected to a series of machining, the end portion of the box portion 20 is expanded in diameter by cold working, and the end portion of the box portion 20 is strengthened by strain aging. In addition, there is a method of reinforcing the end portion of the box portion 20 by induction hardening.

The present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention. For example, the tapered thread constituting the threaded portion of the threaded joint is not limited to the dovetail-type tapered thread, and the type of tapered thread is not limited as long as it is a tapered thread. For example, the tapered thread may be a buttress tapered thread as shown in fig. 4 and 5. The tapered thread shown in fig. 4 is of a type in which the

thread crest surfaces

11a, 21a and the thread groove bottom surfaces 11b, 21b are inclined with respect to the pipe axis CL. The tapered thread shown in fig. 5 is of a type in which the

thread crest surfaces

11a, 21a and the thread groove bottom surfaces 11b, 21b are parallel to the pipe axis CL.

In addition, the threaded joint of the present embodiment can include a shoulder surface in the vicinity of the inner seal in an auxiliary manner. Specifically, the pin portion includes a secondary shoulder surface at the top end. The female snap portion includes an auxiliary shoulder surface corresponding to the auxiliary shoulder surface of the male snap portion. The auxiliary shoulder surfaces are in press-contact with each other in a state where the fastening is completed, and perform the same function as the shoulder surface in the vicinity of the outer seal.

In the threaded joint according to the present embodiment, the threaded portion formed by the male thread portion of the male engagement portion and the female thread portion of the female engagement portion may be a two-step thread divided into two steps along the pipe axis. In the case of a threaded joint using two-step threads, the pin portion may have an intermediate sealing surface between the male thread portion of the first step and the male thread portion of the second step, and the box portion may have an intermediate sealing surface corresponding to the intermediate sealing surface of the pin portion. The intermediate seal surfaces interfere with each other and are closely fitted to each other in a state where the fastening is completed, and an intermediate seal by surface contact is formed. In the case of a threaded joint using two-step threads, the pin portion may have an intermediate shoulder surface between the male thread portion of the first step and the male thread portion of the second step, and the box portion may have an intermediate shoulder surface corresponding to the intermediate shoulder surface of the pin portion. The intermediate shoulder surfaces are in press-contact with each other in a state where the fastening is completed, and perform the same function as the shoulder surface in the vicinity of the outer seal.

Further, each seal surface constituting the seal by the surface contact (outer seal, inner seal, and intermediate seal) is formed together with other portions by a series of machining. At this time, the feed speed of the tool for machining each sealing surface is set to be lower than the feed speed of the tool for machining other portions. Thus, each seal surface is significantly smoother than the machined surfaces of the other portions.

Fig. 6 and 7 are longitudinal sectional views schematically showing the region of the seal based on the surface contact. The outer seal is illustrated in fig. 6 and 7. As shown in fig. 6 and 7, the seal surfaces 13 and 23 refer not only to the region Sa where they contact each other in the fastened state, but also to the entire regions Sb and Sc where the seal surfaces are finished to be smooth surfaces. That is, the seal surfaces 13 and 23 are regions that slide while contacting each other during tightening (including regions that may slide), and include the entire regions Sb and Sc that are processed to have the same surface accuracy as the region Sa that contacts each other in a state in which tightening is completed. The same applies to the inner seal and the intermediate seal.

When the male snap portion 10 is detached from the female snap portion 20 and the seal surface is observed, the seal portion can be recognized. This is because a trace (a region after forced sliding) of the seal portion (the contact region Sa in a state in which fastening is completed) remains on the seal surface.

Examples

In order to confirm the effect of the present invention, a numerical simulation analysis (FEM analysis) based on the elasto-plastic finite element method was performed.

Test conditions

In the FEM analysis, a plurality of models of the threaded joints for steel pipes shown in fig. 1 were prepared. In these models, the protruding portion of the box portion, the front angle of the land surface, and the length of the annular portion were variously changed. Further, cold work applied to the pipe end was simulated by making full use of a model of a part thereof, and the tensile yield strength in the region of both the protrusion and the seal surface was increased to 110% of the tensile yield strength in the region of the pipe main body of the box. The changed conditions are shown in table 1 below.

TABLE 1

Remarks) minimum value of contact surface pressure: is a relative value in the case where the value of No.4 is set to 1.

Labeling: which means that the conditions specified in the present invention are violated.

The following are common characteristics regarding the material and dimensions of the steel pipe.

Size of steel pipe: 14(inch), 112.6(lb/ft) (355.6 mm outside diameter, 20.32mm wall thickness)

Grade of steel pipe: API Standard Q125 (carbon steel for oil well pipes specified in API 5CT, having a tensile yield strength of 125ksi (862N/mm)²))

Thread shape: the dovetail-shaped tapered thread has a clearance between the crest surface of the male thread portion and the bottom surface of the thread groove of the female thread portion, and between the stabbing flanks. The thread height is about 2mm and the thread pitch is 8.47mm

Sealing surface: taper of 10% and contact length of 4mm

Width of shoulder surface: 5mm

Evaluation methodMethod of

In the FEM analysis, the pin part was further tightened 0.01 round from the time when the shoulder surfaces abutted against each other (japanese: ショルダリング) for each model. In this state, a loading procedure simulating the Series a test of ISO136792002 edition (a test in which an internal pressure and an external pressure are repeatedly applied at normal temperature) was sequentially applied to each model. The sealing performance was evaluated by comparing the minimum values of the average contact surface pressures of the sealing surfaces of the outer seal in the inner pressure cycle (first and second quadrants) and the outer pressure cycle (third and fourth quadrants) in the load step history (the higher value means the better sealing performance of the sealing surfaces).

In the evaluation of the sealing performance of the outer seal, the value of test No.4 in which the minimum value of the contact surface pressure of the outer seal was the lowest was set to 1, and the evaluation was performed using the ratio to the test No.4 as an index. As an evaluation criterion, an index of 4 or more was evaluated as good. The results are shown in Table 1 above.

Test results

From the results shown in table 1, the minimum values of the contact surface pressures of the outer seals of test nos. 1 and 2 of the examples of the present invention, which all satisfy the conditions specified by the present invention, were larger than the minimum values of the contact surface pressures of the outer seals of test nos. 3 to 6 of the comparative examples, which do not satisfy any of the conditions specified by the present invention. This proves that the threaded joint for steel pipes of the present embodiment is useful for the sealing performance of the outer seal.

Industrial applicability

The threaded joint of the present invention can be effectively used for connecting steel pipes used as oil country tubular goods.

Description of the reference numerals

10. A male buckle part; 11. an external threaded portion; 11a, a thread crest surface of the external thread part; 11b, a thread groove bottom surface of the external thread part; 11c, an insertion flank of the male screw portion; 11d, a load flank of the male threaded portion; 12. a shoulder surface; 13. a sealing surface; 17. an auxiliary sealing surface; 20. a female buckle part; 21. an internal thread portion; 21a, a thread crest surface of the internal thread part; 21b, a thread groove bottom surface of the internal thread part; 21c, an insertion flank of the female screw portion; 21d, a load flank of the female screw portion; 22. a shoulder surface; 23. a sealing surface; 24. a lip portion; 25. a protrusion; 26. an annular portion; 27. an auxiliary sealing surface; CL, tube axis.

Claims

1. A threaded joint for steel pipes comprising a tubular pin portion and a tubular box portion, the pin portion being screwed into the box portion and the pin portion and the box portion being fastened together, wherein,

the outer diameter of the female buckle part is less than 104% of the outer diameter of the pipe body of the male buckle part,

the female snap part is provided with a lip part and an internal thread part with a taper thread in sequence from the top end to the pipe main body, the lip part comprises a shoulder surface and a taper sealing surface,

the pin part includes: a shoulder surface that comes into contact with the shoulder surface of the female snap portion in a state in which fastening is completed; a sealing surface that comes into contact with the sealing surface of the female snap portion in a fastened state; and an external thread part which is a tapered thread and is fitted into the internal thread part in a fastened state,

the shoulder surfaces of the female buckle part and the male buckle part are inclined at an inclination angle of 5 deg-25 deg from the surface perpendicular to the pipe axis to the screwing-in advancing direction of the male buckle part,

the lip portion of the female snap portion has a protrusion disposed between the shoulder surface and the sealing surface and an annular portion disposed between the sealing surface and the female threaded portion, extending from a distal end side of the female threaded portion in a tube axis direction, and continuous with the sealing surface,

2. A threaded joint for steel pipes according to claim 1,

the male snap portion has a secondary sealing surface at a top end portion,

the female snap portion includes an auxiliary sealing surface that contacts the auxiliary sealing surface of the male snap portion in a state in which fastening is completed.

3. A threaded joint for steel pipes according to claim 1,

the male buckle portion has an auxiliary shoulder surface at a top end,

the female snap portion includes an auxiliary shoulder surface that contacts the auxiliary shoulder surface of the male snap portion in a state where fastening is completed.

4. A threaded joint for steel pipes according to claim 1,

the male screw portion of the male buckle portion and the female screw portion of the female buckle portion are two-segment threads formed by being divided into two segments along a pipe axis.

5. A threaded joint for steel pipes according to claim 4,

the male threaded portion has an intermediate sealing surface between the male threaded portion of the first section and the male threaded portion of the second section,

the female snap portion includes an intermediate sealing surface that contacts the intermediate sealing surface of the male snap portion in a state in which fastening is completed.

6. A threaded joint for steel pipes according to claim 4,

the male threaded portion has an intermediate shoulder surface between the male threaded portion of the first section and the male threaded portion of the second section,

the female snap portion includes an intermediate shoulder surface that comes into contact with the intermediate shoulder surface of the male snap portion in a state in which fastening is completed.

7. A threaded joint for steel pipes according to claim 1,

a tensile yield strength in a circumferential direction in a region of both the protrusion and the sealing surface of the box portion is 105% or more of a tensile yield strength in a region of the tube main body of the box portion.

8. A threaded joint for steel pipes according to claim 1,

in a longitudinal section taken along a pipe axis, a shape of the sealing surface of each of the box portion and the pin portion is one of a straight line, an arc, an elliptic arc, and a quadratic curve, or a combination of two or more of them.

9. A threaded joint for steel pipes according to any of claims 1 to 8 wherein,

the outer diameter of the pipe main body of the male buckle part is more than 170 mm.